Sang-Pil Lee

Origin of negative blood oxygenation level-dependent fMRI signals

Functional magnetic resonance imaging (fMRI) techniques are based on the assumption that changes in spike activity are accompanied by modulation in the blood oxygenation level—dependent (BOLD) signal. In addition to conventional increases in BOLD signals, sustained negative BOLD signal changes are occasionally observed and are thought to reflect a decrease in neural activity. In this study, the source of the negative BOLD signal was investigated using T2*-weighted BOLD and cerebral blood volume (CBV) techniques in isoflurane-anesthetized cats. A positive BOLD signal change was observed in the primary visual cortex (area 18) during visual stimulation, while a prolonged negative BOLD change was detected in the adjacent suprasylvian gyrus containing higher-order visual areas. However, in both regions neurons are known to increase spike activity during visual stimulation. The positive and negative BOLD amplitudes obtained at six spatial-frequency stimuli were highly correlated, and negative BOLD percent changes were approximately one third of the postitive changes. Area 18 with positive BOLD signals experienced an increase in CBV, while regions exhibiting the prolonged negative BOLD signal underwent a decrease in CBV. The CBV changes in area 18 were faster than the BOLD signals from the same corresponding region and the CBV changes in the suprasylvian gyrus. The results support the notion that reallocation of cortical blood resources could overcome a local demand for increased cerebral blood flow induced by increased neural activity. The findings of this study imply that caution should be taken when interpreting the negative BOLD signals as a decrease in neuronal activity.

Diffusion-weighted spin-echo fMRI at 9.4 T: microvascular/tissue contribution to BOLD signal changes.

The nature of vascular contribution to blood oxygenation level dependent (BOLD) contrast used in functional MRI (fMRI) is poorly understood. To investigate vascular contributions at an ultrahigh magnetic field of 9.4 T, diffusion‐weighted fMRI techniques were used in a rat forepaw stimulation model. Tissue and blood T2 values were measured to optimize the echo time for fMRI. The T2 of arterial blood was 40.8 ± 3.4 msec (mean ± SD; n = 5), similar to the tissue T2 of 38.6 ± 2.1 msec (n = 16). In comparison, the T2 of venous blood at an oxygenation level of 79.6 ± 6.1% was 9.2 ± 2.3 msec (n = 11). The optimal spin‐echo time of 40 msec was confirmed from echo‐time dependency fMRI studies. The intravascular contribution was examined using a graded diffusion‐weighted spin‐echo echo‐planar imaging technique with diffusion weighting factor (b) values of up to 1200 sec/mm2. Relative BOLD signal changes induced by forepaw stimulation showed no dependence on the strength or direction of the diffusion‐sensitizing gradients, suggesting that the large vessel contribution to the BOLD signal is negligible at 9.4 T. However, gradient‐echo fMRI performed with bipolar diffusion sensitizing gradients, which suppress intravascular components from large vessels, showed higher percent signal changes in the surface of the brain. This effect was attributed to the extravascular contribution from large vessels. These findings demonstrate that caution should be exercised when interpreting that higher percent changes obtained with gradient‐echo BOLD fMRI are related to stronger neural activation. Magn Reson Med 42:919–928, 1999.

Functional MRI of calcium-dependent synaptic activity: cross correlation with CBF and BOLD measurements.

Spatial specificities of the calcium‐dependent synaptic activity, hemodynamic‐based blood oxygenation level‐dependent (BOLD) and cerebral blood flow (CBF) fMRI were quantitatively compared in the same animals. Calcium‐dependent synaptic activity was imaged by exploiting the manganese ion (Mn++) as a calcium analog and an MRI contrast agent at 9.4 T. Following forepaw stimulation in α‐chloralose anesthetized rat, water T1 of the contralateral forepaw somatosensory cortex (SI) was focally and markedly reduced from 1.99 ± 0.03 sec to 1.30 ± 0.18 sec (mean ± SD, N = 7), resulting from the preferential intracellular Mn++ accumulation. Based on an in vitro calibration, the estimated contralateral somatosensory cortex [Mn++] was ∼100μM, which was 2–5‐fold higher than the neighboring tissue and the ipsilateral SI. Regions with the highest calcium activities were localized around cortical layer IV. Stimulus‐induced BOLD and CBF changes were 3.4 ± 1.6% and 98 ± 33%, respectively. The T1 synaptic activity maps extended along the cortex, whereas the hemodynamic‐based activation maps extended radially along the vessels. Spatial overlaps among the synaptic activity, BOLD, and CBF activation maps showed excellent co‐registrations. The center‐of‐mass offsets between any two activation maps were less than 200 μm, suggesting that hemodynamic‐based fMRI techniques (at least at high field) can be used to accurately map the spatial loci of synaptic activity. Magn Reson Med 43:383–392, 2000.

Simultaneous Blood Oxygenation Level-Dependent and Cerebral Blood Flow Functional Magnetic Resonance Imaging during Forepaw Stimulation in the Rat

The blood oxygenation level-dependent (BOLD) contrast mechanism can be modeled as a complex interplay between CBF, cerebral blood volume (CBV), and CMRO2. Positive BOLD signal changes are presumably caused by CBF changes in excess of increases in CMRO2. Because this uncoupling between CBF and CMRO2 may not always be present, the magnitude of BOLD changes may not be a good index of CBF changes. In this study, the relation between BOLD and CBF was investigated further. Continuous arterial spin labeling was combined with a single-shot, multislice echo-planar imaging to enable simultaneous measurements of BOLD and CBF changes in a well-established model of functional brain activation, the electrical forepaw stimulation of a-chloralose-anesthetized rats. The paradigm consisted of two 18- to 30-second stimulation periods separated by a 1-minute resting interval. Stimulation parameters were optimized by laser Doppler flowmetry. For the same cross-correlation threshold, the BOLD and CBF active maps were centered within the size of one pixel (470 µm). However, the BOLD map was significantly larger than the CBF map. Measurements taken from 15 rats at 9.4 T using a 10-millisecond echo-time showed 3.7 ± 1.7% BOLD and 125.67 ± 81.7% CBF increases in the contralateral somatosensory cortex during the first stimulation, and 2.6 ± 1.2% BOLD and 79.3 ± 43.6% CBF increases during the second stimulation. The correlation coefficient between BOLD and CBF changes was 0.89. The overall temporal correlation coefficient between BOLD and CBF time-courses was 0.97. These results show that under the experimental conditions of the current study, the BOLD signal changes follow the changes in CBF.

Relative changes of cerebral arterial and venous blood volumes during increased cerebral blood flow: implications for BOLD fMRI.

Measurement of cerebral arterial and venous blood volumes during increased cerebral blood flow can provide important information regarding hemodynamic regulation under normal, pathological, and neuronally active conditions. In particular, the change in venous blood volume induced by neural activity is one critical component of the blood oxygenation level‐dependent (BOLD) signal because BOLD contrast is dependent only on venous blood, not arterial blood. Thus, relative venous and arterial blood volume (rCBV) and cerebral blood flow (rCBF) in α‐chlorolase‐anesthetized rats under hypercapnia were measured by novel diffusion‐weighted 19F NMR following an i.v. administration of intravascular tracer, perfluorocarbons, and continuous arterial spin labeling methods, respectively. The relationship between rCBF and total rCBV during hypercapnia was rCBV(total) = rCBF0.40, which is consistent with previous PET measurement in monkeys. This relationship can be linearized in a CBF range of 50–130 ml/100 g/min as ΔrCBV(total)/ ΔrCBF = 0.31 where ΔrCBV and ΔrCBF represent rCBV and rCBF changes. The average arterial volume fraction was 0.25 at a basal condition with CBF of ∼60 ml/100 g/min and increased up to 0.4 during hypercapnia. The change in venous rCBV was 2‐fold smaller than that of total rCBV (ΔrCBV(vein)/ΔrCBF = 0.15), while the arterial rCBV change was 2.5 times larger than that of total rCBV (ΔrCBV(artery)/ΔrCBF = 0.79). These NMR results were confirmed by vessel diameter measurements with in vivo videomicroscopy. The absolute venous blood volume change contributes up to 36% of the total blood volume change during hypercapnia. Our findings provide a quantitative physiological model of BOLD contrast. Magn Reson Med 45:791–800, 2001.

Early temporal characteristics of cerebral blood flow and deoxyhemoglobin changes during somatosensory stimulation.

The close correspondence between neural activity in the brain and cerebral blood flow (CBF) forms the basis for modern functional neuroimaging methods. Yet, the temporal characteristics of hemodynamic changes induced by neuronal activity are not well understood. Recent optical imaging observations of the time course of deoxyhemoglobin (HbR) and oxyhemoglobin have suggested that increases in oxygen consumption after neuronal activation occur earlier and are more spatially localized than the delayed and more diffuse CBF response. Deoxyhemoglobin can be detected by blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI). In the present study, the temporal characteristics of CBF and BOLD changes elicited by somatosensory stimulation in rat were investigated by high-field (9.4 T) MRI. With use of high-temporal-resolution fMRI, it was found that the onset time of the CBF response in the somatosensory cortex was 0.6 ± 0.4 seconds (n = 10). The CBF changes occurred significantly earlier than changes in HbR concentration, which responded after 1.1 ± 0.3 seconds. Furthermore, no early increases in HbR (early negative BOLD signal changes) were observed. These findings argue against the occurrence of an early loss of hemoglobin oxygenation that precedes the rise in CBF and suggest that CBF and oxygen consumption increases may be dynamically coupled in this animal model of neural activation.

MRI assessment of neuropathology in a transgenic mouse model of Alzheimer's disease

The cerebral deposition of amyloid β‐peptide, a central event in Alzheimers disease (AD) pathogenesis, begins several years before the onset of clinical symptoms. Noninvasive detection of AD pathology at this initial stage would facilitate intervention and enhance treatment success. In this study, high‐field MRI was used to detect changes in regional brain MR relaxation times in three types of mice: 1) transgenic mice (PS/APP) carrying both mutant genes for amyloid precursor protein (APP) and presenilin (PS), which have high levels and clear accumulation of β‐amyloid in several brain regions, starting from 10 weeks of age; 2) transgenic mice (PS) carrying only a mutant gene for presenilin (PS), which show subtly elevated levels of Aβ‐peptide without β‐amyloid deposition; and 3) nontransgenic (NTg) littermates as controls. The transverse relaxation time T2, an intrinsic MR parameter thought to reflect impaired cell physiology, was significantly reduced in the hippocampus, cingulate, and retrosplenial cortex, but not the corpus callosum, of PS‐APP mice compared to NTg. No differences in T1 values or proton density were detected between any groups of mice. These results indicate that T2 may be a sensitive marker of abnormalities in this transgenic mouse model of AD. Magn Reson Med 51:794–798, 2004.

Histological co-localization of iron in Abeta plaques of PS/APP transgenic mice.

This study confirms the presence of iron, co-localized with Aβ plaques, in PS/APP mouse brain, using Perls‘ stain for Fe3+ supplemented by 3,3′-diaminobenzidine (DAB) and Aβ immunohistochemistry in histological brains sections fixed with formalin or methacarn. In this study, the fixation process and the slice thickness did not interfere with the Perls’ technique. The presence of iron in ß-amyloid plaques in PS/APP transgenic mice, a model of Alzheimer’s disease (AD) pathology, may explain previous reports of reductions of transverse relaxation time (T2) in MRI studies and represent the source of the intrinsic Aß plaque MR contrast in this model.

Magnetic field correlation imaging

A magnetic resonance imaging (MRI) method is presented for estimating the magnetic field correlation (MFC) associated with magnetic field inhomogeneities (MFIs) within biological tissues. The method utilizes asymmetric spin echoes and is based on a detailed theory for the effect of MFIs on nuclear magnetic resonance (NMR) signal decay. The validity of the method is supported with results from phantom experiments at 1.5 and 3 T, and human brain images obtained at 3 T are shown to demonstrate the methods feasibility. The preliminary results suggest that MFC imaging may be useful for the quantitative assessment of iron within the brain. Magn Reson Med, 2006.

Lower levels of glutathione in the brains of secondary progressive multiple sclerosis patients measured by 1H magnetic resonance chemical shift imaging at 3 T

Background: Disability levels for patients with secondary progressive multiple sclerosis (SPMS) often worsen despite a stable MRI T2 lesion burden. The presence of oxidative stress in the absence of measurable inflammation could help explain this phenomenon. In this study, the assessment of an in vivo marker of oxidative stress, cerebral glutathione (GSH), using magnetic resonance chemical shift imaging (CSI) is described, and GSH levels were compared in patients with SPMS and healthy controls. Objective: To assess whether GSH, a key antioxidant in the brain, is lower in the SPMS patients compared to matched controls. Methods: Seventeen patients with SPMS (Expanded Disability Status Scale = 4.0–7.0; length of MS diagnosis = 19.4 ± 7 years) and 17 age- and gender-matched healthy controls were studied. GSH levels were measured in the fronto-parietal regions of the brain using a specially designed magnetic resonance spectroscopy technique, CSI of GSH, at 3T. Results: The levels of GSH were lower for SPMS patients than for controls, the largest reduction (18.5%) being in the frontal region (p = 0.001). Conclusion: The lower GSH levels in these patients indicate the presence of oxidative stress in SPMS. This process could be at least partially responsible for ongoing functional decline in SPMS.